The American SpaceX company has stacked the biggest rocket ever constructed.
The vehicle's two segments - an upper-stage called Starship and a booster called Super Heavy - were connected together at the firm's Starbase R&D facility in Boca Chica, Texas.
Standing roughly 120m (400ft) in height, the SpaceX rocket dwarfs any previous launch system.
When it eventually lifts off, it will produce about twice the thrust of the vehicles that sent men to the Moon.
The main engines on Apollo's famous Saturn V rockets delivered some 35 meganewtons (nearly 8 million pounds of force) off the pad. The new SpaceX Super Heavy booster should achieve around 70 meganewtons.
A massive crane was needed to join the two segments together. They were held in position for an hour before then being separated again.
SpaceX still has weeks to months of testing ahead of it to prepare for the inaugural flight.
This will see the booster hurl the Starship into space for a once-around-the-Earth trip, which will end with a disposal "landing" in waters off the Hawaiian islands in the Pacific. The Super Heavy will be ditched in the Gulf of Mexico.
Ultimately, though, SpaceX wants both segments of future vehicles to make controlled touchdowns, on land or on sea platforms, so they can be re-used.
Company CEO, Elon Musk, says the Starship system, once fully developed, will be more than capable of taking humans to the Moon and Mars.
It could also ferry people quickly around the globe. Putting satellites in orbit is another obvious application.
The American space agency, Nasa, has already contracted SpaceX to produce a version of the Starship upper-stage that can land astronauts near the lunar south pole this decade.
Friday's milestone follows months of breathtaking activity in Boca Chica.
Engineers have built a succession of prototypes at Starbase. These have helped the design team understand the best way to manufacture the huge stainless steel craft. And by launching some of the Starship iterations on high-altitude hops, SpaceX technicians have also gained invaluable flight experience.
The segments mated briefly on Friday are codenamed B4 (the Super Heavy booster) and Ship 20 (the Starship upper-stage). The pair won't launch until they have a licence from the Federal Aviation Administration (FAA).
The government agency is currently conducting an environmental review that will invite at some point a 30-day public consultation. So, the maiden orbital outing is still some way off. But Mr Musk is very keen that it should happen well before the end of the year.
He described seeing the brief mating of the segments on Friday as a "dream come true".
The most powerful rocket currently in operation is the Falcon Heavy, also built by SpaceX. This produces 23 meganewtons of thrust at launch.